As a typical example of conventional airtight woven fabric sheets for air bags, U.S. Pat. No. 4,977,016 (Japanese Unexamined Patent Publication No. 4-2835) discloses a polyester woven fabric (which will be referred to as a noncoated woven fabric hereinafter) not coated with or not impregnated with a resin and having an air permeability of 0.5 ml/cm.sup.2 /sec/0.5 inch Aq or less. Also, U.S. Pat. No. 5,010,663 (Japanese Unexamined Patent Publication No. 4-2835) discloses a noncoated polyester woven fabric with a permeability of 1.5 ml/cm.sup.2 /sec/0.5 inch Aq or less.
Those polyester woven fabrics are both surface-smoothed woven fabrics produced by applying a calendering operation to both the surfaces thereof. Those woven fabrics obtained by the both surface-calendering operation have a high air-permeability in comparison with that of a resin-coated woven fabric. In examples of the patent specifications, the air permeability is higher than 0.1 ml/cm.sup.2 /sec/0.5 inch Aq.
It has been known that when the above-mentioned conventional woven fabric sheets are practically employed to form an air bag, the above-mentioned value of air permeability causes easy formation of air-permeable perforations in the air bag, and thus the resultant air bag has such a fatal defect that an inflation gas flows out through the perforations and the face of an occupant in automobile or aircraft is burnt by the gas.
Specially, when a large amount of fine particles are generated upon inflating, or when the inflation is carried out using a powerful inflator, it is not too much to say that the above-mentioned noncoated woven fabric air bag has a serious defect. Since the calendering operation is applied to both the surfaces, the resultant woven fabric sheet is provided with both smoothed surfaces thereof and has a high stiffness and thus is disadvantageous in that the fabric is paper-like.
Since the polyester woven fabric has a lower burst strength at sewn portions thereof than that of nylon 66 woven fabric, it is necessary to enhance the burst strength of the sewn portions at, for example, portions surrounding the inflator and top cloth of the air bag, by applying an adhesive reinforcement which has a higher reinforcing effect than that of usual sewing, or by applying a sewing operation after the portions to be sewn are reinforced by the adhesive reinforcement to enhance the burst strength of the portions of the air bag. This is true not only for a beltless bag but also for a belt-attached bag. However, the above-mentioned both surface-calendered woven fabric has a poor bonding property due to the smooth surfaces thereof and thus it is very difficult to reinforce the woven fabric by laminate-adhering a reinforcing fabric, which may be the same as or different from the woven fabric, thereto.
U.S. Pat. No. 4,921,735 (Japanese Unexamined Patent Publication) discloses a woven fabric having an air permeability of 0 to 0.53 ml/cm.sup.2 /sec/0.5 inch Aq and usable as a calendered airtight woven fabric for air bag. However, examples of the U.S. patent do not show any concrete values of the gas permeability. Also, the woven fabric of the U.S. patent has a warp density of 40.6 yarns/inch and a weft density of 86.4 yarns/inch. Since the warp density and the weft density are very different, the resultant woven fabric disadvantageously exhibits not only a significantly lower burst strength in one direction, but also a remarkably poor bonding strength in one direction. Further, as is same for the two afore-mentioned U.S. patents, U.S. Pat. No. 4,921,735 does not disclose a technical idea of smoothing only one surface of the woven fabric and a concept of improving a bonding property of the woven fabric.
Accordingly, an object of the present invention is to solve the above-mentioned problems of conventional non-resin-coated polyester woven fabric for air bags which remain unsolved, and to provide an airtight woven fabric sheet for air bags, having a low air permeability, a specific surface property which allows a surface of the fabric to be adhesive-reinforced, an enhanced burst strength, and an excellent feel, and capable of preventing the formation of gas-permeable perforations in the bag to prevent burns by the inflation gas flowing out through the perforations, and a process for producing the same.